Clarify in the book when rustdoc tests are run

[contradictioned]

The new chapter on documentation is great, but in my opinion the documentation tests are still missleading.

When you

1. create a crate with cargo new --bin mycrate
2. write a doc test
3. run cargo test

it will not run this doctest, only if the create was created without the --bin flag. This behaviour is documented in the crates manifest but this should be noted directly in the book. Particularly as I think, most readers of the book will play around with a project with a main.rs but not with a lib.rs.

[thiagooak]

Why do we run them on cargo test only for libraries?

Would be nice to know in order to write the docs.

That's right, cargo test tests embedded documentation too.

Note that cargo test will run the embedded documentation tests on libraries only, 
we do not run it on binary programs in order to... / because ...

[contradictioned]

I have no idea, in the discussion in the IRC we just did not really have any reasoning. (But I'm a rust noob :) )